Thyroid hormone-induced alterations in membrane structure-function relationships: Studies on kinetic properties of rat kidney microsomal Na ⁺,K⁺-ATPase and lipid/phospholipid profiles

The effects of thyroidectomy (Tx) and subsequent treatment with 3,5,3′-triiodothyronine (T₃) or combined replacement therapy (T_R) with T₃ and thyroxine (T₄) on the substrate and temperature kinetics properties of Na⁺,K ⁺-ATPase and lipid/phospholipid makeup of rat kidney microsomes were examined. Enzyme activity was somewhat high in the hypothyroid (Tx) animals and increased significantly following T₃ treatment, while T_R treatment caused a decrease. In the Tx and T₃ groups enzyme activity resolved in two kinetic components, while in the T_R group the enzyme showed allosteric behavior up to 0.5 mm ATP concentration. The K_m and V_max values of both the components decreased in Tx animals without affecting the catalytic efficiency. T₃ treatment caused a significant increase in the V_max of both the components, with a significant increase in the catalytic efficiency, while the K_m values were not upregulated. The T_R regimen lowered the K_m and V _max of component II but improved the catalytic efficiency. Thyroid status-dependent changes were also noted in the temperature kinetics of the enzyme. Regression analysis revealed that changes in the substrate and temperature kinetics parameters correlated with specific phospholipid components.